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Abstract Rising anthropogenic carbon emissions have dire environmental consequences, necessitating remediative approaches, which includes use of solid sorbents. Here, aminopolymers (poly(ethylene imine) (PEI) and poly(propylene imine) (PPI)) are supported within solid mesoporous MIL‐101(Cr) to examine effects of support defect density on aminopolymer‐MOF interactions for CO₂uptake and stability during uptake‐regeneration cycles. Using simulated flue gas (10 % CO₂in He), MIL‐101(Cr)‐ρ_high(higher defect density) shows 33 % higher uptake capacity per gram adsorbent than MIL‐101(Cr)‐ρ_low(lower defect density) at 308 K, consistent with increased availability of undercoordinated Cr adsorption sites at missing linker defects. Increasing aminopolymer weight loadings (10–50 wt.%) within MIL‐101(Cr)‐ρ_lowand MIL‐101(Cr)‐ρ_highincreases amine efficiencies and CO₂uptake capacities relative to bare MOFs, though both incur CO₂diffusion limitations through confined, viscous polymer phases at higher (40–50 wt.%) loadings. Benchmarked against SBA‐15, lower polymer packing densities (PPI>PEI), weaker and less abundant van der Waals interactions between aminopolymers and pore walls, and open framework topology increase amine efficiencies. Interactions between amines and Cr defect sites incur amine efficiency losses but grant higher thermal and oxidative stability during uptake‐regeneration cycling. Finally, >25 % higher CO₂uptake capacities are achieved for aminopolymer/MIL‐101(Cr)‐ρ_highunder humid conditions, demonstrating promise for realistic applications.